The present invention relates to an agent for radioactively imaging the reticuloendothelial system (RES) of vertebrates, especially primates, particularly the liver, spleen and bone marrow. Such agents are sometimes referred to as radioactive RES imaging agents. More particularly the invention relates to an RES agent comprising a .sup.99m Tc-labelled microaggregated complex of a reducing metal and albumin, particularly human serum albumin (HSA), to the unlabelled microaggregated complex as such and in the form of a kit, to a method of making the same and to a method of using the same for RES imaging.
At one time the most common commercial RES imaging agent was a radio-colloid (particle size of 0.001-0.05 micrometers (.mu.m)) of gold, stablized with gelatin, which, after injection into the blood stream, is removed by and collects in the RES to give a latent radioactive image thereof which can be converted into a visible image by the appropriate instrumentation.
This was displaced by .sup.99m Tc-labelled sulfur colloid stabilized with gelatin, most of which has a particle size of &lt;0.1-1.0 .mu.m, and which is presently still the most widely used radioactive RES imaging agent despite its disadvantages of (a) requiring a relatively large number of components, (b) requiring boiling and neutralization steps for labelling by the user at the use situs, and (c) not being biodegradable. Most of the sulfur colloid RES agents on the market do give sharp clear simultaneous images of the liver and spleen.
A .sup.99m Tc-labelled stannous hydroxide colloid has also been marketed as an RES agent but it has the disadvantage that it is difficult to prevent growth of the colloidal particles after labelling without the subsequent addition of stabilizers by the user at the use situs which make them unsatisfactory for RES imaging, i.e., they are not stable.
Another RES agent which has been marketed in small quantities is a .sup.99m Tc-labelled stannous phytate complex which, it is believed, is converted to an insoluble colloid by calcium in the blood stream, from which it is then removed by the RES. However, with this agent, difficulty has been encountered in obtaining a clinically acceptable clear, sharp image of the spleen simultaneously with the normal healthy liver. Accordingly, the use thereof has not become widespread.